docs: explain how to use SF Docker with an existing project (#156)

README.md

@@ -26,10 +26,11 @@ A [Docker](https://www.docker.com/)-based installer and runtime for the [Symfony
 ## Docs
 
 1. [Build options](docs/build.md)
-2. [Support for extra services](docs/extra-services.md)
-3. [Deploying in production](docs/production.md)
-4. [Installing Xdebug](docs/xdebug.md)
-5. [Troubleshooting](docs/troubleshooting.md)
+2. [Using Symfony Docker with an existing project](docs/existing-project.md)
+3. [Support for extra services](docs/extra-services.md)
+4. [Deploying in production](docs/production.md)
+5. [Installing Xdebug](docs/xdebug.md)
+6. [Troubleshooting](docs/troubleshooting.md)
 
 ## Credits
 

docs/existing-project.md

@@ -0,0 +1,33 @@
+# Installing on an Existing Project
+
+It's also to use Symfony Docker with existing projects!
+
+First, [download this skeleton](https://github.com/dunglas/symfony-docker). If you clone the Git repository, be sure to remove the `.git` repository to prevent conflicts with the `.git` directory already in your existing project.
+
+Then, copy the Docker-related files from the skeleton to your existing project:
+
+    cp -Rp symfony-docker/. my-existing-project/
+
+Enable the Docker support of Symfony Flex:
+
+    composer config --json extra.symfony.docker 'true'
+
+Re-execute the recipes to update the Docker-relatd files according to the packags you use
+
+    rm symfony.lock
+    composer symfony:sync-recipes --force --verbose
+
+Double-check the changes, revert the changes that you don't want to keep:
+
+    git diff
+    ...
+
+Build the Docker images:
+
+    docker-compose build --no-cache --pull
+
+Start the projects!
+
+    docker-compose up -d
+
+Browse `https://localhost`, your Docker configuration is ready!